1. Field of the Invention
The invention relates to a control apparatus and method that controls the operating state of an internal combustion engine. More particularly, the invention relates to an internal combustion engine control apparatus and method that controls the air-fuel ratio according to the oxygen concentration in the exhaust gas.
2. Description of the Related Art
Japanese Patent Application Publication No. 2003-214245 (JP-A-2003-214245), for example, describes a control apparatus that performs air-fuel ratio control using an oxygen concentration sensor provided in an exhaust passage. This control apparatus performs learning control, referred to as atmospheric learning, for learning error in the oxygen concentration sensor that is due to manufacturing variation and deterioration over time and the like.
In this atmospheric learning, when the atmosphere in the exhaust passage becomes an air atmosphere because a fuel cut is performed in the engine, a detection signal output from the oxygen concentration sensor is compared with a reference signal value stored in advance in the control apparatus. This reference signal value corresponds to the oxygen concentration in the atmosphere which is known. The amount that the actual detection value is off from the reference signal value is a value that corresponds to the error inherent in each sensor so it is stored in the control apparatus as a learned value for correcting the error inherent in the sensor.
Also, the control apparatus performs a reduction process of a catalyst by adding fuel into the exhaust gas (hereinafter this fuel will be referred to as “reducing fuel” to avoid confusion with fuel that is normally injected during normal fuel injection). When there is a fuel cut immediately after the reducing fuel has been added, the rate of increase in the oxygen concentration slows from the effect of the reducing fuel remaining in the exhaust gas, resulting in a time lag until the oxygen concentration reaches the concentration level in the atmosphere. Therefore, when a fuel cut starts, the control apparatus determines whether there is a history of reducing fuel having been added within a predetermined period of time immediately before the start of the fuel cut. If there is a history, the control apparatus prohibits atmospheric learning.
In this way, if reducing fuel has been added within a predetermined period of time before the start of a fuel cut, the control apparatus prohibits atmospheric learning. However, the period of time after the reducing fuel has been added until atmospheric learning is possible varies depending on the timing at which the reducing fuel is added and the operating state and the engine and the like. Therefore, simply prohibiting the atmospheric learning, as is done with the control apparatus described above, may result in missed learning opportunities, which reduces learning efficiency.
Also, if the specific value of the predetermined period of time is set low in order to increase the learning efficiency, learning may end up being performed while the effects of the reducing fuel that has been added still remain, and as a result, the learning may be erroneous. In this way, with the control apparatus described above, it is difficult to appropriately set the timing of the atmospheric learning when reducing fuel is added.